This invention relates to a piezoelectric acousitc device used for the warning buzzers of a fire alarm, a theft alarm, a gas detector, or interphone, or general bell, buzzer for watch, etc.
The piezoelectric acoustic device uses a piezoelectric sound generating device, and as shown in FIG. 1, the principle of the piezoelectric sound generating device is such that a piezoelectric element 1 and a vibrating plate 2 are adhered integrally to form a composite plate 5, and the piezoelectric element 1 and vibrating plate 2 of the composite plate 5 are connected to an electric circuit 4 by lead wires 3 and 3' to bend to vibrate the composite plate 5 so as to generate a sound. FIG. 2 shows the conventional piezoelectric sound generating device using this principle. A metal vibrating plate 7 is adhered to either one of silver electrodes 5 and 5' on the front and back of the piezoelectric element 1 by an adhesive 6 so as to form a composite plate. This composite plate is supported in the casing 8 by the nodes 9 of the vibration. This casing 8 is opened at the front side in the direction of thickness of the vibrating plate 7, and is closed at the rear surface by an oscillating circuit 10, and the metal vibrating plate 7 and one electrode 5 are connected to the respective oscillating circuits 10. Numeral 11 represents a power supply of the oscillating circuit 10. Therefore, if an intermittent current is applied from the power supply 11 through the oscillating circuit 10 to the piezoelectric element 1, the composite plate composed of the piezoelectric element 1, vibrating plate 7, and electrodes 5 and 5' is bent to vibrate so that a sound is generated from the opening 12 of the front surface of the casing 8 toward an arrow A.
However, since the rear surface of the casing 8 is closed by the oscillating circuit 10 in this device, the sound generated at the rear side of the composite plate cannot be fed out of the casing 8, and the sound can only be generated from the opening 12 of the front surface of the casing 8, and accordingly only a small sound volume can be obtained. It is considered that a resonant plate 13 is provided at the front of the opening 12 so as to amplify the generated volume still, but large volume cannot be obtained.
The present invention contemplates to eliminate the above disadvantages of the conventional piezoelectric acoustic device, and to provide a novel and improved piezoelectric acoustic device.
It is an object of the present invention to provide a piezoelectric acoustic device which generates a large sound volume with less power of consumption.
According to one aspect of the present invention, there is provided a piezoelectric acoustic device which comprises a composite plate having a piezoelectric element and a vibrating plate integrally adhered to each other, an electric circuit connected to the composite plate so that the composite plate is bent to generate a sound, and a casing for supporting the composite plate and having openings formed at the front and rear surface sides in the direction of thickness of the composite plate.